Respiration in Plants

Written and reviewed by Anjali Sharma, Biology faculty at UniDrill (M.Sc. Botany, Banaras Hindu University · 8 years teaching NCERT Biology). Based on NCERT Biology Class XI, Chapter 12.

Why not just burn the glucose?

Burn a spoonful of glucose and it gives out all its energy at once, mostly as heat. Do that inside a cell and the cell would be cooked. The cell does something smarter: it takes the same glucose apart in many small steps, each controlled by an enzyme, so that the energy comes out in portions small enough to be captured as ATP, the energy currency of the cell. That slow, controlled oxidation is respiration. It is the breaking of C–C bonds of complex compounds through oxidation inside the cell. The compounds oxidised are called respiratory substrates. Carbohydrates are used usually, but proteins, fats and even organic acids can serve under some conditions.

Photosynthesis stores energy in glucose, sucrose and starch. Respiration, which takes place in the cytoplasm and the mitochondria of eukaryotes, is how plants, and everything that eats them, get that energy back out.

How do plants breathe without lungs?

Plants have no special organs for gas exchange. They use stomata in leaves and lenticels in the bark of woody stems. Three reasons explain why this is enough.

The big picture before the details

Respiration of glucose is one story told in three acts, each in a different place. Knowing the sites is half of this chapter.

The path in one line: glucose → glycolysis → pyruvate → link reaction → acetyl-CoA → Krebs cycle → NADH and FADH₂ → ETS → ATP and H₂O.

Glycolysis: the step every cell shares

The word comes from Greek glycos (sugar) and lysis (splitting). The scheme was worked out by Gustav Embden, Otto Meyerhof and J. Parnas, which is why it is called the EMP pathway. It takes place in the cytoplasm of all living organisms, and in anaerobes it is the only respiratory process. A glucose molecule is partly oxidised, through ten enzyme catalysed reactions, to two molecules of pyruvic acid.

In plants the glucose usually comes from sucrose, the end product of photosynthesis, which invertase splits into glucose and fructose. Both sugars are then phosphorylated by hexokinase and enter the pathway.

The arithmetic. Steps 5 to 8 happen twice for each glucose, because one fructose bisphosphate gives two PGAL. So 4 ATP are made and 2 are spent: the net gain is 2 ATP and 2 NADH. NTA asks "net", not "gross".

Substrate level phosphorylation means that a phosphate is moved directly from a substrate to ADP. It is different from the ETS route, where a proton gradient does the work.

Pause and predict: a cell gets fructose instead of glucose. Does it still make a net 2 ATP per sugar?

Check your answer: essentially yes. Fructose is also phosphorylated by hexokinase and joins the pathway at fructose 6 phosphate, so the ATP-using and ATP-making steps afterwards are the same.

Pyruvate has three possible fates

Glycolysis leaves a pyruvate at a fork. With no oxygen, the cell ferments it, either to lactic acid or to ethanol. With oxygen, it goes into the mitochondrion for complete oxidation. So there are three routes: lactic acid fermentation, alcoholic fermentation and aerobic respiration.

Why fermentation wastes most of the energy. Less than 7 per cent of the energy in glucose is released, and not all of it ends up in ATP. The product (alcohol or acid) is also harmful. Yeast poisons itself when ethanol reaches about 13 per cent, which is why naturally fermented drinks stop at about that strength and anything stronger must be distilled.

Beyond the textbook. For the exam: fermentation is partial breakdown of glucose to ethanol or lactic acid under anaerobic conditions, with a net gain of 2 ATP. In reality: the real job of the last step is not to make the product, but to turn NADH back into NAD⁺. Glycolysis needs a supply of NAD⁺ at step 5, and with no oxygen to reoxidise NADH, ethanol or lactate formation is the only way to keep glycolysis going. Why it matters: it explains why both fermentations share the "NADH becomes NAD⁺" line, a favourite statement in assertion-reason questions.

Aerobic respiration: entering the mitochondrion

In eukaryotes, pyruvate is carried into the mitochondrion. Two crucial events follow. First, pyruvate is oxidised completely by removing every hydrogen, leaving three CO₂. This takes place in the matrix. Second, the electrons removed with the hydrogen are passed to molecular oxygen, with ATP being made as they go. This takes place on the inner membrane.

The link reaction. In the matrix, the pyruvate dehydrogenase complex carries out the oxidative decarboxylation of pyruvate:

Pyruvic acid + CoA + NAD⁺ → acetyl-CoA + CO₂ + NADH + H⁺

Coenzyme A and Mg²⁺ are among the cofactors needed. One glucose gives two pyruvates, so this step yields 2 NADH and 2 CO₂ per glucose.

The Krebs (TCA) cycle

Named after Hans Krebs, this cycle turns the acetyl group into CO₂. It begins when acetyl-CoA condenses with oxaloacetic acid (OAA) and water to give citric acid (6C), catalysed by citrate synthase. The cycle regenerates OAA at the end, which is why it is a cycle and why OAA must always be replenished.

So one turn of the cycle gives 3 NADH, 1 FADH₂, 1 ATP (via GTP) and 2 CO₂. Taking the link reaction and the cycle together, the summary for one pyruvate is pyruvic acid + 4 NAD⁺ + FAD⁺ + 2 H₂O + ADP + Pi → 3 CO₂ + 4 NADH + 4 H⁺ + FADH₂ + ATP. Doubling it for one glucose gives 6 CO₂, 8 NADH, 2 FADH₂ and 2 ATP, which accounts for all the carbon of glucose. Notice that only 2 ATP are made directly. The rest of the energy is waiting in the NADH and FADH₂.

Pause and predict: one pyruvate is oxidised completely to three CO₂. Which steps release them?

Check your answer: one in the link reaction, and two in the cycle (at isocitrate → α-ketoglutarate and α-ketoglutarate → succinyl-CoA).

The electron transport system: where the real harvest happens

The ETS sits in the inner mitochondrial membrane. NADH and FADH₂ hand over their electrons, which pass along a chain of carriers, ending at oxygen. The energy released is used to pump protons across the membrane and then to make ATP.

Cytochrome c is a small, mobile protein attached to the outer surface of the inner membrane that shuttles electrons between Complexes III and IV. Ubiquinone is the mobile carrier between I, II and III.

Why oxygen is essential even though it works only at the end. Oxygen is the final acceptor of the electrons. Without it, Complex IV cannot unload its electrons, the whole chain backs up, NADH cannot become NAD⁺ again, and the Krebs cycle stops. Oxygen removes hydrogen from the system and so keeps the whole process moving.

Making ATP: oxidative phosphorylation. Electron flow through Complexes I, III and IV pumps protons from the matrix into the space between the membranes, building a proton gradient. (Complex II does not pump, which is why FADH₂ yields less ATP than NADH.) ATP synthase has an F₀ part, an integral channel in the membrane, and an F₁ part, the headpiece that projects into the matrix and makes ATP. Protons flow back through F₀, and 4 H⁺ pass through F₀ for each ATP made. The energy comes from oxidation and reduction, not from light, hence the name oxidative phosphorylation. For exam calculations, one NADH gives 3 ATP and one FADH₂ gives 2 ATP.

The balance sheet: where 38 ATP comes from

This is an idealised figure. It assumes that the pathways run one after another in an orderly way, that NADH from glycolysis enters the mitochondrion and is oxidised there, that no intermediate is drawn off to make other compounds, and that glucose is the only substrate.

Beyond the textbook. For the exam: aerobic respiration of one glucose gives a net 38 ATP, with 3 ATP per NADH and 2 per FADH₂. In reality: a living cell does not reach this. Moving glycolytic NADH into the mitochondrion and moving ATP and Pi across the membrane both cost energy, and the generally accepted values are about 2.5 ATP per NADH and 1.5 per FADH₂, giving roughly 30 to 32 ATP per glucose. Why it matters: if a CUET option says 30 to 32 ATP and the others say 38, pick the one that matches the values the question gives (3 and 2 ATP means 38).

Fermentation versus aerobic respiration. Fermentation is partial breakdown, aerobic respiration is complete breakdown to CO₂ and H₂O. Fermentation nets 2 ATP per glucose, aerobic respiration far more. NADH is oxidised slowly in fermentation and very vigorously in aerobic respiration.

The amphibolic pathway: a crossroads, not a one way street

Glucose is the favourite substrate, but other food molecules also enter the same pathway at different points.

Because intermediates are also drawn off to build fatty acids and amino acids, the respiratory pathway works in both catabolism and anabolism. That is why it is called an amphibolic pathway rather than just a catabolic one.

Pause and predict: a starving seedling burns its stored fats. At which compound do the fatty acids enter the pathway?

Check your answer: at acetyl-CoA, which then enters the Krebs cycle.

Respiratory quotient (RQ)

RQ = volume of CO₂ evolved ÷ volume of O₂ consumed. It tells you which substrate a tissue is burning, because the ratio depends on how much oxygen the substrate already contains.

Fats have much less oxygen in the molecule than carbohydrates, so they need extra O₂ from outside and the ratio falls below 1. In real cells substrates are mixed, and pure proteins or fats are never the only fuel.

Beyond the textbook. For the exam: learn 1.0, 0.7 and 0.9. In reality: organic acids, which are already rich in oxygen, give an RQ above 1, and so does respiration in which some oxygen is unavailable. Why it matters: a measured RQ somewhere between the standard values means the tissue is burning a mixture.

Exam traps

60-second revision

Where this chapter connects

• Previous: Photosynthesis in Higher Plants makes the glucose that is burned here, and uses chemiosmosis too.
• Next: Plant Growth and Development uses this ATP to power growth, and shows ethylene raising respiration during fruit ripening.
• Class XII link: Microbes in Human Welfare applies fermentation by yeast and lactic acid bacteria in bread, curd and alcohol.